1. Field of the Invention
The present invention relates to an optical fiber for transmitting optical signals, particularly to an optical fiber having an excellent heat resistance and flexural resistance.
2. Description of the Related Art
In the prior art for the plastic optical fibers, polystyrene (PS) or polymethyl-methacrylate (PMMA) are used as the core material. However, since such materials have a glass transition temperature (Tg) as low as 100.degree. C., the core material will be softened and become fluid in a high temperature atmosphere of 100.degree. C. or higher, and therefore, the upper limit of the usable temperature is about 80.degree. C.
As a method for improving such a disadvantage, various plastic optical fibers using a polycarbonate having excellent transparency and a high Tg of about 150.degree. C. have been mentioned (for example, U.S. Pat No. 3,999,834, Japanese Unexamined Patent Publication (Kokai) No. 57-46204 and Japanese Unexamined Patent Publication (Kokai) No. 60-32004). However, according to the method disclosed in U.S. Pat. No. 3,999,834, crosslinking treatment of the clad material is required after the polycarbonate is coated with the clad material, and therefore, a disadvantage is involved in that the preparation process is cumbersome and also a long time is required for the treatment. On the other hand, Japanese Unexamined Patent Publication (Kokai) No. 57-46204 discloses a plastic optical fiber having a polycarbonate coated with a clad material comprising polymethylmethacrylate and so on. However, these clad materials are not sufficient for heat resistance comparing to polycarbonate. And Japanese Unexamined Patent Publication (Kokai) No. 60-32004 discloses a plastic optical fiber having a core material comprising a polycarbonate coated with a clad material comprising 4-methyl-1-pentene, and further, that an unsaturated carboxylic acid such as acrylic acid may be copolymerized for improvement of the interlayer peel-off strength between the polycarbonate and the clad material. Due to poor adhesion between poly (4-methyl-1-pentene) and polycarbonate, a flexural resistance thereof as an optical fiber is inferior and the adhesiveness cannot be improved in some cases by mere copolymerization with an unsaturated carboxylic acid. Thus, under the present situation, an optical fiber having an excellent heat resistance and flexural resistance has not been practically realized.